In a demodulator for use in digital radio communication systems, nonlinear distortions caused by amplifying RF signals have been compensated conventionally. FIG. 9 is a block diagram showing the configuration of a conventional system for compensating for nonlinear distortions. As shown in FIG. 9, the system comprises a nonlinear distortion compensation processing unit 11, a quadrature modulator 13, a high output amplifier 14, a quadrature demodulator 15, and a nonlinear distortion compensation coefficient calculation unit 17. The apparatus shown in FIG. 9 is intended for quadrature amplitude modulation (QAM), and is applied a baseband quasi-synchronization system which is a common digital demodulation system. Further, in the explanation for the operation of the apparatus shown in FIG. 9, typical indications of Ich and Qch are used for in-phase and quadrature components (channels), respectively.
Each element of the apparatus shown in FIG. 9 will be described in detail below. The nonlinear distortion compensation processing unit 11 has a function of performing nonlinear distortion compensations to baseband signals for Ich and Qch input into terminals 1 and 2, respectively, and outputting them. The quadrature modulator 13 has a function of quadrature-modulating baseband signals for Ich and Qch, to which nonlinear distortion compensations have been performed, and outputting the quadrature modulated signals. The high output amplifier 14 has a function of amplifying the quadrature modulated signals and outputting them as modulated signals, which are output to the outside through a terminal 3.
The quadrature demodulator 15 has a function of outputting quadrature demodulated signals I′ch and Q′ch generated by quadrature-demodulating the quadrature modulated signals output from the high output amplifier 14. The nonlinear distortion compensation coefficient calculation unit 17, into which the baseband signals Ich and Qch and the quadrature demodulated signals I′ch and Q′ch are input, has a function of calculating and outputting a nonlinear distortion compensation coefficient based on the inverse characteristics of the nonlinear characteristics corresponding to the amplitude of the baseband signals. The nonlinear distortion compensation processing unit 11 has a function of multiplying the baseband signals Ich and Qch by data of the nonlinear distortion compensation coefficient, and performing a predistortion (addition of the inverse characteristics of the nonlinear distortion characteristics).
Here, brief explanations will be given for nonlinear distortions caused in the high output amplifier 14 with a radio frequency (RF) band, and for an influence of the linear distortions on transmitted signals. Note that the characteristics of the high output amplifier 14 are shown in dB. It is defined that the input level of the high output amplifier is Pi, the output level is Po, the amplification gain is G, and the saturation power level is Psat. Assuming that the high output amplifier 14 has ideal characteristics, a value, obtained by summing the input level Pi and the amplification gain G, is output, unless the output level Po is equal to or exceeds the saturation power level Psat. Thus, the input/output characteristics of the high outuput amplifier 14 is expressed as the following equation (1):                               P          0                =                  {                                                                                          P                    i                                    +                                      G                    ⁡                                          (                                                                        P                          0                                                <                                                  P                          sat                                                                    )                                                                                                                                                                P                    sat                                    ⁡                                      (                                                                  P                        0                                            ≥                                              P                        sat                                                              )                                                                                }                                    (        1        )            
However, if the high output amplifier 14 is configured with practical electric circuits, the output level Po is gradually compressed as the output level Po comes close to the saturation power level Psat, whereby the characteristic difference between the practical amplifier and an ideal amplifier increases. According to a literature, Behavioral Modeling of Nonlinear RF and Mocrowave Device, by Thomas r. Turlington, Artech House, the input/output characteristics of an amplifier, taking into account the compression effect, can be approximated by the following equation (2).                     Po        =                  Pi          +          G          -                      K            ·                                          log                10                            ⁡                              [                                  1                  +                                      10                    ⁢                                          {                                                                        Pi                          +                          G                          -                          Psat                                                K                                            }                                                                      ]                                                                        (        2        )            
Here, K is a positive number, which is an amplitude compression coefficient showing the characteristics of the amplifier. As K increases, the characteristics of the amplifier degrade. In contrast, as K comes close to zero, the characteristics of the amplifier becomes close to the characteristics of an ideal amplifier.
Further, with reference to the equation (2), if defining that the saturation power level Psat is a reference point (0 dB), Pi+G is the operating point of the amplifier, and the output level is Pop, respectively, the relationship between the operating point and the output level of the high output amplifier 14 is shown as the following equation (3):                     Po        =                  Pop          -                      K            ·                                          log                10                            ⁡                              [                                  1                  +                                      10                                          (                                              Pop                        K                                            )                                                                      ]                                                                        (        3        )            
FIG. 10 shows characteristics against output of the operating point of the amplifier, in a case that the vertical axis shows the output level, K→0, and K=3, 5, 7. Here, the horizontal axis shows the operating point of the amplifier, and the vertical axis shows the output level. As shown in FIG. 10, in the case of an ideal amplifier (K→0), the operating point linearly operates until it comes to the saturation power, and once it came to the saturation power, the power is immediately clipped to the saturation point. Further, the Figure shows that as the amplitude compression coefficient K increases, the characteristic difference from the ideal amplifier becomes large, causing the linear operation to be less performed before the operating point level exceeds the saturation point (0 dB).
The modulated signals to be demodulated by the demodulated apparatus, shown in FIG. 9, are quadrature amplitude modulated (QAM) signals in which signal points have plural amplitudes. Therefore, when the aforementioned nonlinear operation occurs, there arises an influence of nonlinear distortions with different compressibility on each signal point, corresponding to the signal amplitude.
FIG. 11 shows an example of a signal point arrangement of quadrature amplitude modulated signals to be demodulated by the demodulator shown in FIG. 9. FIG. 11(a) shows a normal signal point arrangement of QAM signals which are sixteen quadrature amplitude modulated signals, and FIG. 11(b) shows a signal point arrangement, in which only a first quadrant in the signal arrangement shown in FIG. 11(a) is extracted. It should be noted that in FIGS. 11(a) and 11(b), black circles show signal points, and + symbols show normal positions of the signal points. FIGS. 11(a) and 11(b) show the state where the black circles and + symbols of the signal points overlap with each other.
As shown in FIG. 11(a), the normal signal point arrangements of the sixteen quadrature amplitude modulated signals exist on first to forth quadrants defined by the horizontal axis Ich and the vertical axis Qch, with four points having the same amplitude, respectively, in the same manner. Hereinafter, the signal point arrangement will be explained with respect to the first quadrant only, since the amplitudes are same in the second to the forth quadrants and the operations thereof are also similar. Further, the four points in the signal point arrangement of the first quadrant are named point A, point B, point C and point D, respectively, for convenience.
FIG. 12 shows a signal point arrangement on the first quadrant when the sixteen quadrature amplitude modulated signals are affected by a nonlinear distortion. Here, the black circles also represent signal points and the + symbols also represent normal positions of the signal points. FIG. 12 shows a state where the black circles showing the signal points are shifted from the + symbols showing the signal points due to a nonlinear distortion.
As shown in FIG. 12, the outside signals (points A, B and C) with larger amplitudes are more affected by the nonlinear distortion, comparing with the inside signal (point C) having a smaller amplitude, whereby the shifted amounts of the outside signals from the normal signal point positions indicated by the + symbols are large. In particular, the signal point on the outermost side with the largest amplitude, as the point B, is shifted significantly.
When demodulating such a signal, the margin between the demodulated signal point and the judgment area is small, whereby the outer side signal is more affected by noise, and the error rate becomes worse. Note that the boundaries shown by dashed lines in FIG. 12 are divisions of the signal judgment areas.
When expressing the equation (3) by assigning a variable x as the operating point power and a generalized function F(x) as the output power, the following equation (4) is established:                               Po          =                      F            ⁡                          (              Pop              )                                      ,                              F            ⁡                          (              X              )                                =                      X            -                          K              ·                                                log                  10                                ⁡                                  [                                      1                    +                                          10                                              (                                                  X                          K                                                )                                                                              ]                                                                                        (        4        )            
When using the inverse function of F(x), the following equation (5), in which the input is the output power and the output is the operation point power, inversely, can be established:Pop=F−1(Po)  (5)
It is difficult to express the inverse function, used in the equation (5), as a formula. However, since Pop and Po in the equation (4) have a relationship of one to one, the relationship in the equation (5) can be expressed in FIG. 13, using a numerical calculation by assigning a parameter K. In FIG. 13, nonlinear distortions caused in the RF amplifier are expressed as the characteristics of the operating point level (dB) with reference to the output level (dB). The output level shown by the horizontal axis corresponds to the input power into the nonlinear distortion compensation processing unit 11, and the operation point level shown by the vertical axis corresponds to the output level after the predistortion is performed.
Further, when defining the amplitude ratio of the output power to the operating point power as the amplitude compensation rate Re, and the amplitude of the output power as the input amplitude of the quadrature demodulator 15, the amplitude compensation rate characteristics of the quadrature demodulator 15 to the input amplitude can be expressed as shown in FIG. 14.
In FIG. 14, the horizontal axis shows the ratio to the amplitude of the saturation power in dB. Thereby, if the operating points of the output signals of the high output amplifier can be estimated, the amplitude compensation rate can be calculated by converting the input amplitude of the nonlinear distortion compensation processing unit 11 into dB. Hereinafter, the operating point of the average signal power is defined as the average operating point. An influence of the amplitude distortions can be compensated by tracking the average operating point corresponding to the adaptive operation, calculating the amplitude compensation rate according to the operating point of the average signal power detected and the input amplitude of the nonlinear distortion compensation processing unit 11, and multiplying the input signals by the amplitude compensation rate.
FIG. 15 is a block diagram showing the configuration of the nonlinear distortion compensation coefficient calculation unit 17. As shown in FIG. 15, the nonlinear distortion compensation coefficient calculation unit 17 comprises: a root-sum-square calculation circuit 23, an amplitude compensation rate calculation table processing circuit 24, an average operating point estimation circuit 25, a compensation polarity detection circuit 26, a judgment circuit 27, terminals 56 to 59, and a terminal 60. To the terminals 56 and 57, baseband signals Ich and Qch are input, respectively. The root-sum-square calculation circuit 23 outputs signals, which are resultants of calculating root sum square of the amplitude of the respective baseband signals Ich and Qch, as an input amplitude to the amplitude compensation rate calculation table processing circuit 24.
To the terminals 58 and 59, quadrature demodulated signals Ich′ and Qch′ are input, respectively. The judgment circuit 27 judges a transmitted symbol based on the quadrature demodulated signals Ich′ and Qch′, and generates and outputs data signals and error signals. The compensation polarity detection circuit 26, corresponding to the result of judging the appropriateness of the amplitude distortion compensation based on the compensation polarity area defined by the boundary where the vector of the input error signal becomes perpendicular to the vector of the data signal, generates a control signal for adjusting the compensation amount in the amplitude distortion compensation, and output it to the average operating point estimation circuit 25.
FIG. 16 is a chart showing an example of a compensation polarity detection area for detecting an influence of distortion from a signal point arrangement. The compensation polarity detection circuit 26 estimates an area where data exist according to the data signals output from the judgment circuit 27, and detects an influence of distortions based on judgment reference data which vary according to the area where the signals exist, and on the error signals. More specifically, in the compensation polarity detection circuit 26, the origin 0 of the signal point arrangement and the normal signal point position are linked by a straight line, and a straight line perpendicular to the straight line is set as a boundary, and by judging the inside of the boundary (origin 0 side, no shading) as an area affecting the positive nonlinear distortion, and by judging the outside of the boundary (shaded parts) as an area affecting the negative nonlinear distortion, an adaptive operation is performed so that the both judged areas are generated with the equal possibility.
The average operating point estimation circuit 25 generates and outputs an average operation point estimation value by adaptively changing it, corresponding to the control signal output from the compensation polarity detection circuit 26. The amplitude compensation rate calculation table processing circuit 24 has an amplitude compensation rate calculation table, with which an amplitude compensation rate can be delivered by assigning an average operating point estimated value output from the average operating point estimation circuit 25 and an input amplitude output from the root-sum-square calculation circuit 23. The amplitude compensation rate calculation table processing circuit 24 outputs data of amplitude compensation rate, delivered from the table, which data is output to the outside through the terminal 60.
FIG. 17 is a block diagram showing the configuration of the nonlinear distortion compensation processing unit 11. As shown in FIG. 17, the nonlinear distortion compensation processing unit 11 comprises terminal 51 to 55, and two multipliers 21. To the terminals 51 and 52, baseband signals Ich and Qch from the terminals 1 and 2 are input, respectively. To the terminal 55, an amplitude compensation rate signal is input. Respective multipliers 21 multiply the input baseband signals Ich and Qch by the amplitude compensation rate signal, respectively, and output the resultant signals. The resultant signals output from the multipliers 21 are output through the terminals 53 and 54, respectively.
However, the nonlinear distortion compensation operation described above uses the result obtained by a numerical calculation based on the expected characteristics of the high output amplifier 14. Thereby, there has been a problem that expected nonlinear distortion compensation characteristics may not be exhibited when the operation is affected by incompleteness of the amplitude delay characteristics or an intrinsic deviation, that the quadrature modulator 13 or the high output amplifier 14 have.
In other words, since a conventional demodulator performs only nonlinear compensations on the transmitter side, linear distortions caused by the incompleteness of the amplitude delay characteristics or intrinsic deviation of an analog circuit are not reduced. Thereby, there has been a problem that an effect of the nonlinear distortion compensation characteristics may not be fully exhibited due to the influence of the linear distortions.
It is therefore an object of the invention to provide a transmitter, capable of performing an ideal nonlinear distortion compensation, in which an influence of incompleteness of the amplitude delay characteristics or an intrinsic deviation, that an amplifier has, is eliminated.